Base load generators such as nuclear power plants and generators with stochastic, intermittent energy sources, such as wind turbines and solar panels, generate excess electrical power during times of low power demand. Large-scale electrical energy storage systems are a means of diverting this excess energy to times of peak demand and balance the overall electricity generation and consumption.
In EP 1577548, the applicant has described the concept of a thermoelectric energy storage (TEES) system. A TEES converts excess electricity to heat in a charging cycle, stores the heat, and converts the heat back to electricity in a discharging cycle, when necessary. Such an energy storage system is robust, compact, site independent and is suited to the storage of electrical energy in large amounts. Thermal energy can be stored in the form of sensible heat via a change in temperature or in the form of latent heat via a change of phase or a combination of both. The storage medium for the sensible heat can be a solid, liquid, or gas. The storage medium for the latent heat occurs via a change of phase and can involve any of these phases or a combination of them in series or in parallel.
The round-trip efficiency of an electrical energy storage system can be defined as the percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage. Round-trip efficiency is increased when thermodynamic reversibility factors are maximized. However, it is important to point out that all electric energy storage technologies inherently have a limited round-trip efficiency. Thus, for every unit of electrical energy used to charge the storage, only a certain percentage is recovered as electrical energy upon discharge. The rest of the electrical energy is lost. If, for example, the heat being stored in a TEES system is provided through resistor heaters, it has approximately 40% round-trip efficiency. The efficiency of thermoelectric energy storage is limited for various reasons rooted in the second law of thermodynamics. Firstly, the conversion of heat to mechanical work in a heat engine is limited to the Carnot efficiency. Secondly, the coefficient of performance of any heat pump declines with increased difference between input and output temperature levels. Thirdly, any heat flow from a working fluid to a thermal storage and vice versa requires a temperature difference in order to happen. This fact inevitably degrades the temperature level and thus the capability of the heat to do work.
It is also noted that the charging cycle of a TEES system is also referred to as a heat pump cycle and the discharging cycle of a TEES system is also referred to as a heat engine cycle. In the TEES concept, heat needs to be transferred from a hot working fluid to a thermal storage medium during the heat pump cycle and back from the thermal storage medium to the working fluid during the heat engine cycle. A heat pump requires work to move thermal energy from a cold source to a warmer heat sink. Since the amount of energy deposited at the hot side is greater than the work required by an amount equal to the energy taken from the cold side, a heat pump will “multiply” the heat as compared to resistive heat generation. The ratio of heat output to work input is called coefficient of performance, and it is a value larger than one. In this way, the use of a heat pump will increase the round-trip efficiency of a TEES system.
In EP 08162614, the applicant has described the concept of utilizing transcritical thermodynamic cycles to improve TEES systems. FIG. 1 illustrates temperature profiles in a heat exchanger in contact with a thermal storage medium during charging and discharging cycles of a transcritical TEES system. The abscissa represents the provided heat in the system, the ordinate represents the temperature, and the lines on the graph are isobars. The solid line indicates the temperature profile of the working fluid in a transcritical TEES charging cycle. The dotted line indicates the temperature profile of the working fluid in a transcritical TEES discharging cycle. The straight diagonal dashed line indicates the temperature profile of the thermal storage medium in a transcritical TEES cycle. Heat can only flow from a higher to a lower temperature. Consequently, the characteristic profile for the working fluid during cooling in the charging cycle has to be above the characteristic profile for the thermal storage media, which in turn has to be above the characteristic profile for the working fluid during heating in the discharging cycle. The temperature profiles are stationary in time due to the sensible heat storage in the thermal storage medium. Thus, while the volume of thermal storage medium in the heat exchanger remains constant, the volume of the hot and cold thermal storage medium stored in the hot-fluid and cold-fluid storage tanks changes. Also, the temperature distribution in the heat exchanger remains constant.
A transcritical cycle is defined as a thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. There is no distinction between a gas phase and a vapor phase beyond the critical point and therefore there is no evaporation or boiling (in the regular meaning) in the transcritical cycle.
It is established that the transfer of heat over large temperature differences is a thermodynamic irreversibility factor. In FIG. 1, the maximum temperature difference, ΔTmax, between the thermal storage medium and the working fluid on discharging and the minimum temperature difference, ΔTmin, between the thermal storage medium and the working fluid on charging, are both indicated. In order to minimize the maximum temperature difference ΔTmax, relatively large heat exchangers could be constructed or phase change materials can be used for thermal storage. Problematically, these solutions result in a high capital cost and therefore are not generally practical.
Furthermore, even if relatively large heat exchangers are used, the thermodynamic properties of the working fluid act to limit the minimization of temperature differences. This is shown by the curvature in the working fluid temperature profiles (isobars) in FIG. 1. The curvature results in an “internal pinch point” and increases the average temperature difference regardless of the size of the heat exchanger. “Pinch analysis” is a known methodology for minimizing energy consumption in processing systems by determining thermodynamically feasible heat exchange networks.
As would be apparent to the person skilled in the art, a reduction of heat transfer losses are of particular importance in the considered application of thermal energy storage with charging based on a heat pump cycle. In such an application, any increase of heat exchange temperature losses during charging and discharging directly translates to loss of useful work and reduction of the round trip efficiency of the system.
Thus, there is a desire to provide an efficient thermoelectric energy storage having a high round-trip efficiency and a minimal approach temperature, while minimizing the amount of required thermal storage medium, and also minimizing the capital cost.